The fundamental components of a dynamic positioning (DP) system are: one or more position reference systems to measure the vessel position and heading; thrusters to apply control action; and a controller to determine the required thrusts. The object of a DP system is not to hold the vessel absolutely stationary, but to maintain its station within acceptable limits. The magnitude of the permitted position variation is dependent upon the application and on operational concerns. In many applications a loss of position beyond the acceptable limits may have a severe impact either on the safety of personnel or equipment, or on the environment. It is vital, therefore, that adequate measures are taken to maintain the integrity of the DP system as far as is reasonably possible.
The DP system combines all available measurements of position, from whatever source, into a single estimate of vessel position. The process of combining the available measurements can be referred to as ‘data fusion’.
The sources of measurements can include a wide variety of position measurement equipment (PME) such as gyrocompasses (which offer compact, reliable and accurate measurement of vessel heading (yaw), independent of outside disturbances), taut wires, satellite navigation systems (which include global positioning systems (GPS) and differential GPS (DGPS)), inertial navigation systems (INS), and hydro-acoustic positioning systems.
Different PME typically provide different accuracy of measurement data. The main difference is the level of noise on the measurement data. For example, a DGPS system might provide position measurements with noise of 0.3 m standard deviation, whereas a hydro-acoustic positioning system might provide position measurements with noise of 5 m standard deviation. Because the DP system will normally need to use all of the available measurement data for redundancy purposes it must carry out a data fusion process. Typically the data fusion process will use some form of weighting based on the accuracy of the measurement data provided by each PME. This ensures that a PME that provides lower quality measurement data (i.e. a noisy PME) will have less of an effect on the position estimate derived by the data fusion process than a PME that provides higher quality measurement data.
The accuracy of the measurement data provided by a particular PME may not always be constant. For example, the noise on the position measurements provided by a DGPS will depend on the number and spread of satellites that are visible to the receiver at any particular time. The noise on the position measurements provided by a hydro-acoustic positioning system will depend on the water depth and the number of transponders deployed on the seabed. Known DP systems therefore maintain an estimate of the accuracy of the measurement data provided by each PME in order to determine the relative weightings for the data fusion process. This accuracy estimate is typically calculated using measurement data collected over a period of a minute or more. Because it relies on previous position measurements, the accuracy estimate is slow to respond to sudden changes in the behaviour of the PME, e.g. when the receiver of a DPGS system loses sight of a number of satellites due to shading.